1. Field of the Invention
The present invention relates generally to plasma diagnostic methods for semiconductor wafer processing systems. More particularly, the present invention relates to methods relating to wafer integrated plasma probe assembly arrays.
2. Description of the Related Art
In the semiconductor industry, plasma, generally comprising of partially ionized gas, is employed in etching and deposition processes whereby films are etched from or deposited onto wafer surfaces. In these processes, plasma can be characterized in terms of characteristics of the interaction between the surface to be processed and the plasma. Such characteristics, and measure quantities of the characteristics, are important in order to control the etch or deposition rate and consequently, the desired dimension of the etch depth or deposited film. These characteristics include the rate of flow of charged particles impinging upon the surface to be processed, the potential distribution of the plasma, the ion current flux, the electron temperature and density, and the ion energy.
In plasma etching systems, knowledge of the potential distribution of the plasma is useful because the energy with which particles impinge upon the surface to be processed depends upon the potential distribution. In addition, the plasma potential determines the energy with which ions strike other surfaces in the chamber. High-energy bombardment of these surfaces can cause sputtering and consequent re-deposition of the sputtered material upon the surface to be processed. In addition, process uniformity is related to the uniformity of the plasma.
Similarly, the ion current flux is an important characteristic of the plasma generated within a reaction chamber of a semiconductor wafer processing system. This characteristic generally defines the effectiveness of the semiconductor wafer processing system. Specifically, the ion current flux affects the uniformity of the etch process and indicates potential damage to a wafer. The measurement of ion current at various locations within the chamber is therefore important to characterize the effectiveness of the plasma in processing a wafer.
It is thus desirable to diagnose instantaneously from outside the processing chamber the various characteristics of the interaction between the surface to be processed and the plasma. Prior art FIG. 1A is an illustration showing a conventional wafer 1 having probe structures 2 formed thereon. The conventional wafer 1 consists of a Si wafer with the probe structures 2 fabricated using three levels of masks such as substrate contact, metal pad and oxide layer. The wafer 1 is processed using conventional wafer manufacturing techniques.
Prior art FIG. 1B shows the probe structures 2 in greater detail. The probe structure 2 includes a semiconductor substrate layer 4, which is on the semiconductor wafer 1 on which are comb-like structures 6 made from metals such as Cu or aluminum and with or without layers of insulators such as oxide layers 8. In the fabrication of semiconductor IC""s in which advanced MOS devices require multiple levels of metal interconnections, the size of the comb-like structures is such that the height of the structure could be less than 0.5 micron and the space between the structures could be less than 0.4 micron wide such that the aspect ratio could be greater than two. The aspect ratio is defined as the height of the comb-like structure divided by the width of the space between the comb-like structures of prior art FIG. 1B. The presence of tall structures on the substrate of a semiconductor wafer sometimes causes a differential charging of the surface due to the difference in electron and ion currents crossing the plasma sheath to the closely spaced structures.
The differential charging of the surface (prior art FIG. 1C) is mostly indicative of a non-uniform plasma which includes fluctuations in the electron and ion densities, and also indicates differences in surface potentials and charge flux densities. If plasma is non-uniform it is anticipated that the depth of etching or the depth of deposition would be variant across the surface of the wafer. Differential charging also could cause oxide damage in semiconductor devices due to differences in charge flux densities. This is very important as plasma is in contact with smooth and not so smooth surfaces on the wafer.
It is a purpose of the plasma diagnostics to ensure that the plasma is uniform across the wafer surface so that the different processes taking place in the plasma chamber would result in high yield for the device output.
Prior art FIG. 1C is an illustration showing a probe structure 2 on a conventional wafer 1. As shown in prior art FIG. 1C, the presence of probe structures causes shaded regions 10 where there is charge accumulation and unshaded regions 12 where there is no charge accumulation. The sign of the charge depends on the surface potential of the structure. Local inequality of positive and negative charge fluxes reaching the wafer surface results in a net charge. Local charge-flux imbalances result in circulating currents through the wafer that generate charging damage in gate oxides as in IC process equipment.
This calls for application of sufficient RF power for better gap fill capability. If the plasma is not uniform across the substrate, then the resulting current imbalance causes a voltage to build up in the substrate. This voltage allows the current from the plasma to flow in the substrate to the gate oxides of underlying MOS transistors. However, application of sufficient RF power could cause damage to the gate oxides leading to gate leakage or oxide breakdown when the amount of current exceeds the capacity of the gate oxide.
In general, there are two conventional methods of diagnosing the characteristics of interest, a probing method, and an electromagnetic wave method. In the probing method, a probe 200 may comprise an electrode 204 (usually made from metal, prior art FIG. 2A) on a support 206 and directly introduced into the plasma 202 to detect the electric current in the plasma, which is then analyzed to determine the characteristics of the plasma. The probe 200 is also called the Langmuir probe. A graph 250 with a characteristic I-V curve 252 (prior art FIG. 2B) is obtained by varying the voltage on the electrode 204 and measuring the current when the probe 200 is placed in the plasma. The I-V curve 252 indicates that for a large negative value of the probe potential, all electrons are essentially repelled and only ions contribute to the current leading to an ion saturation current (Isat). This ion saturation current Isat simply determines the electron density provided electron temperature can be determined. Conversely, Isat is also a product of electron charge, disk surface area and ion flow.
In the electromagnetic wave method, electromagnetic waves (including microwaves and lasers beams) interact with the plasma and the results of the interaction are detected. By way of example, a beam reflected from the plasma is detected by spectroscopy and analyzed.
The probing method is limited to probing plasma of relatively low temperature and density. For plasmas of electron density Ne on the order of 10{circumflex over ( )}14 cm-3 and above and electron temperature of a few tens of electron volts and above, the probing method is of limited use. The electromagnetic method suffers from being complex and expensive to manufacture.
In view of the limitations of prior art Langmuir probes and probe structures that are also charge monitors, what is needed is a method of making and using diagnostic tools capable of taking simultaneous measurements of plasma characteristics such as uniformity, electron or ion flux densities, potentials and ion energy in real time across a wide area of the wafer surface while the wafer is inside of the plasma chamber.
A preferred embodiment of the present invention includes a method of making an array of electrical probes formed upon an upper surface of a semiconductor wafer. In use, the array of electrical probes provides simultaneous measurement of plasma characteristics in real time across a wide area of the wafer surface. The plasma is diagnosed while in the process chamber to study characteristics of the plasma as it interacts with a wafer. The plasma may be tested, for example, for being homogenous in its electron or ion flux density, potential and particle temperature.
The planar array of plasma probes or the planar plasma probe assembly array is connected to the connectors on the wafer through the conductive interconnects. The resultant assembly of probe assembly arrays, conductive interconnects and the connectors form a wafer Integrated planar plasma probe assembly array. The probe assemblies are preferably arranged in a pattern: one probe assembly in the center and four more probe assemblies at intermediate positions such that the four probe assemblies lie along the radius from the center to the corners; the corners being the four corners of a square box near the edge of the wafer. On the same wafer are located four optional plasma probe assemblies spaced from the existing probe assemblies such that they lie roughly in between the probe assembly at the center and probe assemblies at the corners of a square. Each probe has six possible probe elements. The probe elements are wafer integrated Langmuir probes. The probe elements are, however, made from low impedance N-type silicon and are exposed to the plasma unlike in prior art where the Langmuir probe elements are conductors.
In a method of the present invention, the probe elements are clustered into an assembly such that four of the six probe elements are of intermediate size or medium size, shaped roughly like squares, and are charge monitors with patterning on them. The structures of the four medium sized probe elements have a non-zero aspect ratio. The four medium sized probe elements are suitable for patterning in different ways to diagnose potentials due to charge shading effects. The probe structures on the four medium sized square elements are rectangular comb-like structures, which don""t necessarily have identical aspect ratio. An important aspect of having a range of aspect ratios for the probe elements is that it gives an idea in real time as to what aspect ratio would cause wafer damage in real time. The presence of probe structures on probe elements determines charge accumulation from the difference in electron and ion currents as they cross the plasma sheath to reach the plasma structures on the wafer substrate. Such a differential electron or ion flux from non-uniform plasmas is responsible for causing charge induced damage in some semiconductor devices. The fifth probe element has an area equal to the four medium sized probe elements and has no patterning on it. An absence of patterning makes the probe a plain probe. The fifth probe element with no patterning is considered to have a zero aspect ratio. The fifth probe element is considered as a reference Langmuir probe and is exposed to the plasma for measurements of floating potential and saturated ion flux. The sixth probe element is a plain probe with no patterning on it. Again, an absence of patterning constitutes zero aspect ratio. The sixth probe element is capable of providing electron measurements. However, the sixth probe element is very much smaller than any probe element or exposed substrate for the reason that in order to perform electron measurements, the excess current could cause damage to the probe if the probe""s area is bigger because the probes are essentially made of conductors. The sixth probe element is a novel addition to any of the prior art in plasma diagnostics as it allows electron measurements along with flux, potential and charging damage measurements performed simultaneously in real time.
While it is important that the geometrical areas of the probe elements be such that the probe elements form a probe assembly, it is not a requirement that areas of probe pads be the same for all the probes. The geometrical shapes of the probe elements are also not a critical requirement for the invention. In the invention, the area of the smallest probe pad is about 0.25 mm squared while the area of the medium sized probe elements having square pads is 25 square mm each.
The probe pads, the conductive interconnects and the connectors are all placed on an N-type silicon wafer. There is also a large area that is not used for any probing purposes and is exposed to the plasma. These vacant areas on the substrate are covered with a low impedance N-type silicon for the sole purpose of making an ohmic contact easy. The large area of the wafer substrate also acts a floating reference electrode. At the floating potential, the probe collects both the saturation-ion current as well as canceling electron current such that the net current through the probe is zero.
There are also four more optional plasma probe assemblies arranged in between the center probe assembly and the corner probe assemblies. The four intermediate plasma probe assemblies can be rotated with respect to the plasma probe assemblies located already at the center and at the corners of the wafer.
Connections from the probe assemblies on the substrate to connectors are made on wafer traces. By arranging the connector pads to conform to a standardized mass termination array it is relatively convenient to connect them using wire bonds to a flexible circuit jumper strip to get the signals off of the wafer and into external diagnostic circuitry which includes an analyzer. The analyzer measures the relative electron or ion potentials and current flows from the charge particle fluxes, energies and impressed voltages. The probe assemblies on the wafer surface measure the plasma charge densities and energies when the plasma comes in contact in the plasma processing chamber. The local grouping of probe array assemblies at nine places allows both spatial resolution and real time measurement of six quantities: DC potential, AC potential, shading induced potentials, ion fluxes, ion energy distribution, and the electron component of the I-V Langmuir probe characteristic simultaneously.
Such an arrangement of planar probe assembly arrays determines electron or ion flux densities, potentials and ion energy in real time across a wide area of the wafer surface while the wafer is inside of the plasma chamber. The probe assemblies on the wafer allow six different measurements on the wafer when the plasma is in the charge shaded region or when it is in the charge unshaded region.
In wafer processing, it is highly preferred that the deposition or etching induced by plasma be uniform because millions of devices get built on a single wafer. As there is need to manufacture more number of devices on a single large wafer to reduce costs, it is imperative that the process involved be as uniform as possible. The diagnostics from such semiconductor equipment should indicate the quality of plasma over a wider area in the semiconductor process chamber because that would ultimately determine the device quality.
For a fuller understanding of the nature and advantages of the present invention, reference is made to the following detailed description taken together with the accompanying figures.